1. Field of the Invention
The present invention relates generally to foundry sand of the type used to make core molds into which non-ferrous metals are cast and, more specifically, to a foundry sand and method for providing cores and molds having significantly improved shake-out and collapsibility characteristics.
2. Description of the Prior Art
Foundry sand is commonly used to make core and molds into which ferrous and non-ferrous metals are cast. The core/molds consist of sand bonded with special additives, including inorganic binders such as clay and organic or "resin" binders, such as phenol, melamine or urea formaldehyde. The term "chemically bonded cores" is intended in the context of the present discussion to encompass both organic and resin bonded foundry sands of the type described above.
One known process for making metal castings employing resin bonded sands is the "shell process." In this process, a core or mold is formed in the desired configuration from the resin-coated sand and a metal is then poured around the shell cores. The resin system slowly burns out, removing the resin binder from the system. If the system works satisfactorily, the core collapses.
An example of a previously known resin binder used in the shell process is a phenolic novolak resin cured with hexamethylene tetramine. These particular resin binders have obtained widespread acceptance because of resulting high tensile strengths, with the result being the formation of very strong cores. One disadvantage of the use of such resin binders occurs as a result of the incomplete decomposition of the resin binder during the casting process. If the resin binder burns out more or less completely or decomposes, the core which remains consists essentially of sand and becomes free flowing and can be readily removed from the casting. On the other hand, if the chemically bonded sand does not degrade to a sufficient extent, the core or portions thereof remain inside the casting and can generally only be removed by mechanical means. The result is that in some applications, such as engine blocks and heads, it is virtually impossible to remove pieces of the core which have not been completely burned out and collapsed.
One particular problem with the prior art resin binders concerns the manufacture of aluminum alloy casting of the type used in automobile engine manufacture. Iron and steel are generally poured at temperatures in the range from about 2200.degree. to 3000.degree. F. At those temperatures, burnout of the chemically bonded sands is usually complete. However, aluminum, brass, bronze and other metals and alloys have lower melting points and are poured at temperatures on the order of 1200.degree. to 2000.degree. F. These lower temperatures present the possibility of additional problems with shake out and collapsibility of the core. At such lower temperatures, resin bonded sands, in particular, do not burn out completely resulting in cores or portions of cores being left within the castings.
Prior methods used in attempting to remove organic bonded foundry cores by mechanical means, such as attrition during airless blast machine cleaning of the castings, vibrating the castings through their natural frequency range to disintegrate the core remaining in the internal cavities or manually using pneumatic impact tools, have only been economically successful in certain instances. The core removal stage of casting cleaning still remains a major cost factor in the manufacture of cast products. Particularly aluminum alloy castings manufactured in so-called semi-permanent molds, consisting of a metal or graphite mold into which resin bonded cores are set.
In order to facilitate removal of cores by any of the aforementioned methods, the principal breakdown or collapsibility control of foundry cores has been to adjust the proportions of the ingredients in the mix from which the cores are made. For instance, in conventional baked oil sand cores, the amount of binder oil and combustible material such as wood flour or cereal are varied as well as the baking time. A decrease in the amount of binder oil and an increase in the amount of wood flour, lowers the temperature at which the cores breakdown after the molten metal has been poured. It also lowers the tensile strength of the core and may result in mechanical failure of the cores under metallic static head pressure at the beginning of the molten metal pour.
Collapsibility of resin bonded cores can only be adjusted by increasing or decreasing the binder resin and catalyst within narrow limits because these components of the mix also control the core making process in terms of "working time" and "stripping time" in the case of cold curing systems. Gas cured systems demand critical control of both resin binder and gassing time in order to maintain physical properties within consistent narrow limits. Hot cured systems are based on a "time-temperature" function that is related to the type of resin such as phenolic, urea, a blend of the two resins, or a urethane. They are all thermosetting therefore, curing temperature is critical.
As the need to reduce the weight of automobiles in order to meet the mileage standards of the Federal Rules and Regulations becomes more urgent in the late nineties, the foundry industry is converting more and more production capacity to semi-permanent molding of thin-walled aluminum alloy castings, particularly ceramic powder reinforced aluminum and magnesium super alloys.
This trend has intensified the problem of core breakdown or collapsibility due to the reduced mass of the cast metal available for burning the organic binders in the cores to a weak "carbon bond" to facilitate removal during the casting cleaning stage. If the cores retain a high percentage of their original cured tensile strength, removal by any mechanical means results in casting distortion, breakage or disintegration and uneconomical labor costs.
One object of the present invention is to provide a method for reducing the physical strength of the residual chemical binder in a mold or core to a weak carbon bond between the sand grains by means of self-sustained oxidation, after the molten metal has been poured.
Another object of the invention is to ensure uniform collapsibility of the mold or core during the elapsed time of pouring the molten metal and shake-out or ejection from a permanent mold or die.
A further object of the invention is to control the rate of collapsibility of the mold or core without affecting its specified physical properties, prior to the start of the molten metal pouring stage.
A further object of the invention is to control the rate of collapsibility of the mold or core without affecting its specified permeability in terms of volume and rate of gas evolution during the molten metal pouring and cooling stages.
Another object of the invention is to control the rate and uniformity of collapsibility of the mold or core to enable the casting cleaning stage to be automated.